Frame rate control (FRC) is a common technique used to increase the perceived color depth or the number of quantized levels of an electronic image signal over the original quantization limit of the electronic image signal. In the simplest form, an intermediate image grey level in between two neighboring signal quantization levels is obtained by displaying the two neighboring levels in alternate frames, thus providing an averaged grey level in visual perception. This FRC method can theoretically be carried out to display more grey levels in between by controlling the on/off ratios of the two neighboring levels over a number of frames, except for the undesirable visual flickering when pixels are designated on and off at below 50 Hz rates.
Dithering is another common technique used to increase the perceived color resolution over the signal quantization resolution. Intermediate grey or color levels are created by varying the proportion of black and white pixels or of pixels of different available quantized colors spatially. Dithering will not reduce spatial resolution and the dithered image will remain smooth and sharp as long as the dithering dimensions are below visual perception. Dithering can be done using a regular predefined pattern, a random pattern, or a pseudo random pattern. FRC and dithering can also be used together, providing an additional degree of flexibility. The dithering pattern can be changed from frame to frame.
In a conventional frame rate control system, there is color loss in high tone colors, because the generation of color sub-levels is available only between quantization levels of output signals. For example, a 6-bit TFT display panel has 64 quantization levels of an output signal in each color channel. Applying a conventional method of frame rate control can only generate 3 sub-levels between two adjacent quantization levels, making a total number of 253 levels. The total number of colors can be displayed by such a method is 253 (red)×253 (green)×253 (blue)=16.2 million. A true 8-bit TFT display panel, which has a total number of 256 quantization levels of an output signal in each color channel, can display a total number of 16.7 Million colors. Therefore, comparing with a true 8-bit TFT display panel, the 6-bit display panel with the conventional frame rate control system has a color loss of about 0.5 million colors. The color loss happens in the highest 3 levels in each color channel.